schoolswikiaorg-20200215-history
Nuclear Fission
Just Imagine... It's an early day in August in 1945. You're a fighter pilot for the United States airforce and you've been assigned to a secret mission. You are told to take the plane "The Enola Gay," and fly it over Japan and deliver a payload over the city of Hiroshima. You have been told that you had better drop your payload and get out of the area fast, because there's going to be a huge explosion. So you, Brigadier General Paul W. Tibbets, deliver the bomb over Hiroshima and fly away as quickly as possible. A minute later, and you turn around-- A huge mushroom-shaped cloud has appeared, reaching high into the sky. It is no normal cloud- it is the direct result of the Nuclear bomb you just dropped, which has vaporized nearly the whole city of Hiroshima and instantly killed hundreds of thousands. You wonder, "Wow, how could anyone make a bomb with such powerful effects?" The answer to that question is Nuclear Fission. The Enola Gay Official Site http://www.radshelters4u.com/nuke-bomb1.jpg This picture was taken from www.radshelters4u.com/nuke-bomb1.jpg ---- A Little Bit of History.... Origin of the Word Atom The word atom is borrowed from the Greek language. The prefix "a" means "not" and the Greek word "tomos" means "cuttable." So the literal translation of the word "atom" from Greek to English is "uncuttable," meaning it was believed to be the smallest possible unit of matter (matter is anything that takes up space). Fritz, Strassman, and Fermi Until the 1930s, nobody had really thought of trying to split the atom, until the Italian Physicist Enrico Fermi and the German physicists Otto Hahn and Fritz Strassman made some very important discoveries and theories. http://www.bartleby.com/images/A4images/A4atom.jpg This picture was taken from www.bartleby.com/images/A4images/A4atom.jpg In 1938, Otto Hahn and Fritz Strassman beamed neutrons at a Uranium atom using Fermi's theory that neutrons should be fired at the atoms because they would not be repelled by the nucleus of the atom, which has a positive charge (Don't forget that the nucleus, or center of an atom, is made of protons, which have positive charges, while neutrons have no charge. Also don't forget that things with the same charge repel each other-- If you hold two magnets together with similar poles facing each other, they will repel each other). So when Hahn and Strassman beamed all these neutrons at Uranium atoms, they noticed that some of the atoms were cut in half, and that the reaction caused more neutrons to zoom around. In addition to the many neutrons zooming around, A LOT of energy was produced. When the atom is split into many tinier pieces, some of the matter is "lost" and converted into energy, according to Albert Einstein's famous equation: E=mc^2 ---- Nuclear Chain Reactions So what happens when all those loose neutrons are zooming around the other atoms? The answer is that they collide with the other atoms, and if they're going at just the right speed, they'll cause another nuclear fission reaction...and another...and another...just like dominoes. This repeated process of fission is called a "Nuclear Chain Reaction." A great illustration of this process is found in the following picture, which was found at www.converdyn.com/industry/images/chain-reaction.gif http://www.converdyn.com/industry/images/chain-reaction.gif A Nuclear Chain reaction, as we can see by the huge nuclear explosion diagrammed in the beginning, produces A LOT of energy. In fact, there's so much energy that many countries, including the United States, use controlled Nuclear Chain reactions to produce huge amounts of electricity in order to power homes, offices, buildings, etc. How do you make a controlled nuclear reaction and avoid a huge mushroom cloud over a big city? Enrico Fermi toyed with that idea whie at the University of Chicago. He thought that by throwing neutron-absorbing rods into the mix that he could control the amount of reactions by inserting/pulling out the rods. It turns out he was right. Many nuclear power plants use rods made of Boron, the 5th element on the periodic table, and Cadmium, the 48th element on the table. ---- The "Other" Kind of Fission So far we've gone over Nuclear fission reactions that humans make by shooting neutrons at atoms. In addition to human-made fission reactions, there are fission reactions that happen by themselves and are called "spontaneous Fission reactions." But that name is too long- the alternate scientific name happens to be much shorter and to the point: Decay. There are 2 important types of decay: Alpha decay and Beta Decay. Alpha is a Greek letter of the alphabet, and looks like this: http://www.w3.org/TR/2000/CR-MathML2-20001113/glyphs/003/U003B1.png Beta on the other hand, looks like this: http://www.w3.org/TR/2000/CR-MathML2-20001113/glyphs/003/U003B2.png 'Alpha Decay' Alpha decay is a type of "spontaneous nuclear reaction" that happens when an unstable atom starts breaking off "alpha particles," which are made of 2 neutrons and 2 protons. While this may be a small particle that's broken off, it turns the original atom into a whole new Element!!! For example, Uranium-238 has a mass of 238 a.m.u (atomic mass units) and 92 protons before it undergoes decay. Since Uranium emits alpha particles, when it undergoes decay, it loses 4 a.m.u and has 2 less protons, bringing its atomic number down to 90. Now it's no longer Uranium 238-- It's become Thorium-234, which has a mass of 234 a.m.u and an atomic number of 90. 'Beta Decay' While some elements emit alpha particles, others emit "Beta Particles." There are 2 types of Beta Decay: Beta-Minus (β-) and Beta-Plus (β+). β- Decay In Beta-minus decay, one of the unstable atom's neutrons (remember, they're the particles that have no charge and have the same mass as a proton) turns into a proton, an electron, and this thing called an antineutrino. But let's not really worry about that last one- it isn't vital to understanding the general concept. Since the neutron broke up into a proton and an electron, the number of protons in the atom goes up and changes the atom completely into the atom next to it (to the Right) on the periodic table! Since that neutron became a proton and an electron, the mass of the atom is the same since protons and neutrons have equal masses, and an electron's mass is so small it's considered almost nothing (it's mass is actually 1,837 times smaller than that of a proton or neutron). β+ Decay In Beta-plus decay, one of the atom's protons turns into a neutron, a positron, and a neutrino. What's a positron you ask? A positron is a POSI'tively charged elec'TRON. Because the proton turned into a neutron, the atomic number of the atom went down one, and the mass didn't change at all. Because the number of protons is one less than before, the atom is transformed into the atom immediately to the left of it on the periodic table! Helpful Decay-table with Half- Lives (This table is an abbreviated version of the one found in the NYS Regents Chemistry Reference Tables) Isotopes You may have noticed that in the above table, there's a column labeled "Element (Isotope)." But what exactly is an isotope? It's simply an atom of a certain element with a prescribed mass. For example, there are 2 isotopes of the element Uranium on the table (they're in the same cell). One is Uranium-235, and the other is Uranium-238. Uranium 235 has the same number protons as does Uranium 238, but it has less neutrons, and therefore less mass. Uranium-238 has a mass of 238 a.m.u (atomic mass units) and Uranium-238 has a mass of 235 a.m.u. Half-Life You also may have been scratching your head at what that thing entitled "half-life" is. Half-life is how long it takes for one-half of the atom to decay, which makes sense with the name "half-life." An atom undergoes multiple half-lives until there is almost no remaining portion of the original mass left (in theory, an element can undergo an infinite number of lives- think of it like a piece of cake: you can continually cut the cake in half an infinite number of times...but it will never reach 0. It will get REALLY close though). Looking at the table, you can see that some elements have really long half-lives, such as Uranium-235. Others, on the other hand, have a really short half-life, such as Calcium-37. In fact, in the blink of an eye, most of Calcium-37 is already gone! A pretty simple equation to figure out how much of a given element is left after "n" number of half-lives: Final Mass=Initial Mass/(2^n) Antiparticles and those Funny Things We've Never Heard of A few paragraphs ago, we talked about neutrinos, antineutrinos, and positrons. A neutrino is kind of like a "neutral electron." While an electron has a negative charge, a neutrino has none. A neutrino has almost no mass just like the electron. That of course is an "electron-neutrino." There are two other types of neutrinos that are related to other funky particles called muons and taus. So now you know what a neutrino is. that brings up the next question- what's an antineutrino? An antineitrino is an "antiparticle" to the neutrino. What the heck is an antiparticle you ask? an antiparticle of a normal particle is essentially the normal particle with an opposite charge or sign attached to it. For example, a positron is the antiparticle to the electron and has a positive charge attached to it. An antineutrino is the antiparticle to the neutrino. It is also believed to play a role in the conservation of momentum and energy. See Momentum: Collisions and Conservation of Energy. Antiparticles and their regular-particle-partners, when they meet/collide, are transformed into pure energy. SOURCES http://www.atomicarchive.com http://www.lbl.gov/abc/Basic.html http://www.ps.uci.edu/~superk/neutrino.html http://www.palaeos.com/Geochronology/gcglossary.html Barron's Regents Review for Physics Glencoe Physics Textbook http://library.thinkquest.org/17940/texts/fission/fission.html Encyclopedia Britannica http://library.thinkquest.org/17940/texts/fission/fission.html (Great for the Basics of Nuclear fission) www.lanl.gov (Los Alamos National Lab site) http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/fission.html Images www.radshelters4u.com/nuke-bomb1.jpg (The Mushroom Cloud/ Nuclear Explosion) www.bartleby.com/images/A4images/A4atom.jpg (diagram of atom) www.converdyn.com/industry/images/chain-reaction.gif (Nuclear Chain Reaction Diagram) www.w3.org/TR/2000/CR-MathML2-20001113/glyphs/003/U003B1.png (picture of letter alpha) www.w3.org/TR/2000/CR-MathML2-20001113/glyphs/003/U003B2.png (picture of letter Beta)